In an active matrix substrate, thin-film transistors (hereinafter also referred to as “TFTs”), for example, are provided as switching elements in respective pixels, each of which is the smallest unit of an image.
A typical bottom-gate TFT is provided with a gate electrode disposed on an insulating substrate, a gate insulating film disposed so as to cover the gate electrode, a semiconductor layer disposed on the gate insulating film in an island shape so as to overlap the gate electrode, and a source electrode and a drain electrode disposed on the semiconductor layer so as to face each other, for example.
In a typical display device in which peripheral circuits are integrated, thin-film transistors that are used for the switching elements of pixels are required to have a low leak current, and thin-film transistors that are used for the peripheral circuits are required to have a low threshold voltage and capability of high-speed drive, for example.
In fabricating a peripheral circuit that uses a plurality of thin-film transistors, in view of high-speed drive, a CMOS inverter or an enhancement/depletion (E/D) inverter is widely used. The CMOS inverter requires both an n-channel type and a p-channel type. The enhancement/depletion (E/D) inverter is constituted of two thin-film transistors, which have a large difference in respective threshold voltages.
In an active matrix substrate in recent years, for a switching element in each pixel, which is the smallest unit of an image, a TFT using a semiconductor layer made of an oxide semiconductor (hereinafter also referred to as “oxide semiconductor layer”) has been proposed to replace a conventional thin-film transistor that uses an amorphous silicon semiconductor layer. This oxide semiconductor layer is formed of an IGZO (In—Ga—Zn—O) semiconductor film that has a high-speed mobility.
Many oxide semiconductors with high-speed mobility such as amorphous IGZO show n-type (electron) conduction, and are not changed to p-type (hole) conduction even by doping, and thus, a CMOS circuit structure cannot be employed. Therefore, a circuit that uses the oxide semiconductor with high-speed mobility has a problem that a CMOS inverter circuit cannot be used, and in order to achieve such a circuit, it is necessary to fabricate an E/D inverter circuit that can independently control threshold voltages of the respective thin-film transistors and that is capable of high-speed operation.
An E/D inverter that is constituted of thin-film transistors each having an oxide semiconductor as a channel layer has been disclosed. More specifically, disclosed is an E/D inverter in which a channel layer of a first thin-film transistor and a channel layer of a second thin-film transistor have mutually different thicknesses, and at least one of the respective channel layers of the first thin-film transistor and of the second thin-film transistor has been heat-treated. It is described that, in such a configuration, by a difference in thickness between the respective channel layers of the first and second thin-film transistors that constitute the E/D inverter, or by a difference in conditions of heat treatment performed on the channel layer, a difference in the threshold voltage can be created, and therefore, it is possible to make a sufficiently large difference between the respective threshold voltages of two thin-film transistors that constitute the E/D inverter (see Patent Document 1, for example).